1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a device and method for unfastening and lifting a top nozzle subassembly from a reconstitutable fuel assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 (Re. 31,583) and 3,814,667 to Klumb et al and U.S. Pat. No. 4,269,661 to Kmonk et al.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Convention reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life.
One reconstitutable fuel assembly construction, devised recently, is illustrated and described in the last U.S. patent application cross-referenced above. It incorporates a top nozzle subassembly which can be removed from and replaced onto the fuel assembly as a unit.
The top nozzle subassembly includes an upper hold-down plate, an enclosure having a lower adapter plate, a plurality of elongated tubular sleeves disposed between the upper and lower plates, and a plurality of hold-down springs extending between the plates and about the respective sleeves. The upper hold-down plate has a plurality of passageways defined therethrough, while the lower adapter plate has a plurality of openings, the passageways and openings being arranged in respective patterns which are matched to that of the guide thimbles of the fuel assembly. The elongated sleeves are slidably inserted at their respective upper ends into the respective passageways of the upper hold-down plate and at their lower ends are releasably threaded to the upper ends of the guide thimbles. The lower ends of the sleeves cooperate with lower retainers attached to the guide thimbles below the lower adapter plate in holding the adapter plate at a stationary position on the guide thimbles. The sleeves slide axially in the upper hold-down plate passageways to accommodate thermal growth between the fuel assembly and upper core plate.
When all of the sleeves are unthreaded from the upper ends of the guide thimbles, the top nozzle is in condition for removal from the remainder of the fuel assembly for reconstitution thereof. Due to the cross-sectional size of each sleeve and due to a slidable interconnection between the upper hold-down plate and the enclosure of the top nozzle which limits movement of the plates away from one another, each sleeve remains captured between the slidably interconnected upper and lower plates of the top nozzle. The elongated sleeve is hollow so that, in addition to accommodating insertion of a control rod through it, a suitable tool can be inserted into the sleeve for gripping it internally to rotate it in either direction for threading on or unthreading from the upper end of the guide thimble.
While the reconstitutable fuel assembly construction briefly described above has demonstrated considerable promise as a measure by which domestic and foreign utilities can minimize both operating and maintenance expenses, a need exists for means to effectively and efficiently carry out removal and replacement of the top nozzle subassembly of the reconstitutable fuel assembly so as to enhance commercial acceptance thereof.